1. Field of the Invention
The present invention relates to an image forming apparatus for printers, facsimiles, copying machines and so on to form an image with an electrophotographic technique. More particularly, it relates to a technique for charging its photosensitive body with a corona discharger.
2. Related Art
Generally, an image forming apparatus using an electrophotography technology includes a photosensitive body having a photosensitive layer at an outer peripheral face thereof, a charging unit for uniformly charging the outer peripheral face of the photosensitive body, an exposing unit for forming an electrostatic latent image by selectively exposing the outer peripheral face charged uniformly by the charging unit, a developing unit for constituting a visible image (toner image) by providing a toner which is a developing agent to the electrostatic latent image formed by the exposing unit and a transcribing unit for transcribing the toner image developed by the developing unit onto a record member of sheet or the like which is a transcribing object.
There is known charging unit for charging the outer peripheral face of the photosensitive body utilizing a corona discharger referred to as a scorotron charger. The scorotron charger includes a discharge electrode, a supporting member for supporting the discharge electrode, a back plate for carrying out stable discharge and a grid for controlling charge potential on the photosensitive body. When charging is carried out, for example, by applying a voltage of −4KV through −6KV to the discharge electrode, applying −600V (potential dependent on potential intended to charge actually) to a grid and grounding the back plate or applying a potential the same as that of the grid to the back plate, corona discharge is generated from the discharge electrode and the photosensitive body can be charged to about −600V.
Since the scorotron charger as described above uses corona discharge, generation of ozone is inevitable. Ozone is known to degrade the photosensitive body and the charger, resulting in an inferior image formation.
Accordingly, the conventional chargers have a blast aperture on a back side thereof extending along the axial direction of the charger and a blast duct is provided on the back face side, so that the ozone is discharged from the charger with air supplied from one end of the duct (for example, refer to JP-H06-43815-Y2, p. 1, FIG. 1).
In the related art described above, however, as the ventilation of ozone is insufficient, partial degradation of the discharge electrode gradually promotes, causing non-uniform electricity discharging in a low temperature and low humidity environment. As a result, an inferior image with so-called “charging unevenness” is formed.
The inventors have investigated the cause and found that it is due to partial residence of ozone in the charger (particularly, on the downstream side of the air flow).
It has also been found that in order to solve the problem, it is desirable that the back plate has a vent aperture on the side face of the charger (for example, on the side face at the downstream side of the air flow) to efficiently discharge the ozone from the charger.
On the other hand, it has also been found that providing the vent aperture on the side face of the back plate decreases the absolute value of charge potential on the photosensitive body at the corresponding area to the vent aperture. Thus uniformity of the charge potential is deteriorated. For example, when an aperture of about 8 mm in width and 50 mm in length is provided on the side face of the back plate, the absolute value of the charge potential is decreased by about 20V. This is not a negligible difference in view of the recent requirement for high-quality color image. In general, in order to obtain a preferable color image in the image forming apparatus, it is desirable that the in-plane variation in charge potential (variation in the axial direction of the photosensitive body) is not larger than 20V. However, it is difficult to achieve such the uniformity due to the influence of the tolerances of components constituting the charger. Therefore, under such situations, there arises a serious drawback if the charger is configured to have a potential difference about 20V in the axial direction of the photosensitive body in the initial state.